Calibration of combined acceleration and pressure sensors

ABSTRACT

Techniques are disclosed relating to calibrating sensors configured to measure both pressure and acceleration. In various embodiments, a system detects a first voltage produce by a first piezoelectric material in a hydrophone when the hydrophone is exposed to an acceleration and detects a second voltage produced by a second piezoelectric material in the hydrophone when the hydrophone is exposed to the acceleration. The system, in some embodiments, compares the first voltage and the second voltage. Based on the comparing of the first and second voltages, in some embodiments, the system determines a resistance for a variable resistor coupled to one of the first and second piezoelectric materials.

RELATED APPLICATIONS

The present application claims priority to U.S. Provisional Appl. Nos.62/500,537 filed May 3, 2017 and 62/510,972 filed May 25, 2017, whichare herein incorporated by reference in its entirety.

BACKGROUND Technical Field

This disclosure relates generally to marine geophysical surveys and,more specifically, to seismic sensors for use in marine geophysicalsurveys.

Description of the Related Art

A seismic vessel may perform marine geophysical surveys in whichinformation about a local geology is collected and analyzed to determinepossible oil and gas reservoirs. To conduct a survey, a seismic vesseloften tows seismic sources, such as air guns, and sensor streamersthrough various bodies of water. These seismic sources produce acousticenergy waves that can penetrate the seabed and reflect off the rockformations below. The sensor streamers may include multiple sensors,such as hydrophones, that detect the energy waves reflected back by therock formations. These detected waves may be digitized and sent to theseismic vessel where they may be recorded and later used to build amodel of the earth's strata in the local geology.

In many instances, it may be beneficial to collect data pertaining tothe pressure and the particle motion experienced by the streamers.Traditionally, streamers have relied on hydrophones for detectingpressure produced by energy waves and geophones for detecting waterparticle motion. Being able to accurately measure motion and pressuremay be useful in ensuring more accurate readings about the earth'sstrata. For example, measuring pressure and motion may allow “ghost”reflections from the water surface to be removed from the seismic data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a vessel conducting a seismicsurvey using an array of towed submerged streamers, according to someembodiments.

FIG. 2 is a block diagram illustrating exemplary elements of a sensorsystem that is configured to measure pressure and acceleration,according to some embodiments.

FIG. 3A is a block diagram illustrating exemplary elements of a sensorpair wired to cancel acceleration noise, according to some embodiments.

FIG. 3B is a block diagram illustrating exemplary elements of a sensorpair wired to cancel pressure noise, according to some embodiments.

FIG. 4 is a block diagram illustrating exemplary elements of acalibration circuit, according to some embodiments.

FIG. 5 is a block diagram illustrating exemplary elements of an array ofsensor pairs coupled to amplifiers, according to some embodiments.

FIG. 6 is a block diagram illustrating exemplary elements of a sensorsystem that is configured to measure pressure and acceleration,according to some embodiments.

FIGS. 7-9 are flow diagrams illustrating exemplary methods forcalibrating sensors configured to measure pressure and acceleration,according to some embodiments.

FIGS. 10-12 are flow diagrams illustrating exemplary methods formeasuring both pressure and acceleration using the same sensors,according to some embodiments.

This disclosure includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

Within this disclosure, different entities (which may variously bereferred to as “units,” “circuits,” other components, etc.) may bedescribed or claimed as “configured” to perform one or more tasks oroperations. This formulation—[entity] configured to [perform one or moretasks]—is used herein to refer to structure (i.e., something physical,such as an electronic circuit). More specifically, this formulation isused to indicate that this structure is arranged to perform the one ormore tasks during operation. A structure can be said to be “configuredto” perform some task even if the structure is not currently beingoperated. An “amplifier structure configured to amplify a signal” isintended to cover, for example, an integrated circuit that has circuitrythat performs this function during operation, even if the integratedcircuit in question is not currently being used (e.g., a power supply isnot connected to it). Thus, an entity described or recited as“configured to” perform some task refers to something physical, such asa device, circuit, memory storing program instructions executable toimplement the task, etc. This phrase is not used herein to refer tosomething intangible. Thus, the “configured to” construct is not usedherein to refer to a software entity such as an application programminginterface (API).

The term “configured to” is not intended to mean “configurable to.” Anunprogrammed FPGA, for example, would not be considered to be“configured to” perform some specific function, although it may be“configurable to” perform that function and may be “configured to”perform the function after programming.

Reciting in the appended claims that a structure is “configured to”perform one or more tasks is expressly intended not to invoke 35 U.S.C.§ 112(f) for that claim element. Accordingly, none of the claims in thisapplication as filed are intended to be interpreted as havingmeans-plus-function elements. Should Applicant wish to invoke Section112(f) during prosecution, it will recite claim elements using the“means for” [performing a function] construct.

As used herein, the terms “first,” “second,” etc. are used as labels fornouns that they precede, and do not imply any type of ordering (e.g.,spatial, temporal, logical, etc.) unless specifically stated. Forexample, in a structure having eight amplifiers, the terms “first” and“second” amplifiers can used to refer any two of the eight amplifiers.

As used herein, the term “based on” is used to describe one or morefactors that affect a determination. This term does not foreclose thepossibility that additional factors may affect a determination. That is,a determination may be solely based on specified factors or based on thespecified factors as well as other, unspecified factors. Consider thephrase “determine A based on B.” This phrase specifies that B is afactor used to determine A or that affects the determination of A. Thisphrase does not foreclose that the determination of A may also be basedon some other factor, such as C. This phrase is also intended to coveran embodiment in which A is determined based solely on B. As usedherein, the phrase “based on” is thus synonymous with the phrase “basedat least in part on.”

DETAILED DESCRIPTION

As mentioned above, seismic sensor streamers may use two different typesof sensors: a hydrophone for detecting pressure and a geophone fordetecting particle motion. Having different types of sensors fordetecting pressure and motion, however, has its drawbacks. First, usingtwo types of sensors increases the number of sensors in seismicstreamers, which may be quite lengthy, and thus complicates maintainingthe streamers. Second, it can be difficult to isolate one component(e.g., motion) from the other (e.g., pressure) when attempting tomeasure motion and pressure components. For example, when detectingpressure, a hydrophone may experience motion due to the towing vibrationof the streamer, causing the hydrophone to produce an output indicativeof pressure when it is in fact noise from the motion.

The present disclosure describes embodiments in which a system isconfigured to detect both pressure and particle motion in a manner thatovercomes some of the downsides of the prior approaches. In variousembodiments described below, a stream may include multiple hydrophonesensor pairs having sensors with piezoelectric materials. In such anembodiment, the sensor pairs are arranged in a manner that allowspressure and particle motion in the form of acceleration to be measuredsimultaneously using the same sensors, as opposed to relying on twodistinct sensor types for simultaneous measurement. (Although variousreferences are made below to a sensor pair measuring acceleration, notethat such a sensor pair can also be said to measure velocity, which canbe determined by integrating acceleration over time.) In someembodiments, the arrangement of a sensor pair also allows for thepressure component and the acceleration component to be easilyseparated. In particular, the sensor pairs may be coupled to a pair ofdifferential amplifiers in a manner that produces two output signals.The two output signals may then be combined (e.g., through addition) toproduce one component (e.g., pressure) and also simultaneously combined(e.g., through subtraction) to produce the other component (e.g.,acceleration). As will also be discussed, in some embodiments, a sensorpair also may be coupled to a calibration circuit that can improvepressure and acceleration measurements by accounting for discrepanciesthat may exist between the piezoelectric materials present in thesensors.

Turning now to FIG. 1, a vessel conducting a seismic survey using anarray of towed submersible streamers is shown. In the illustratedembodiment, seismic vessel 110 is towing an array of submersible signalsources 130 and an array of submersible streamers 140 that are coupledto vessel 110 via multiple tow cables 120. As shown, streamers 140include pressure and acceleration sensor systems 105. In variousembodiments, the seismic survey may be implemented differently thanshown—e.g., one seismic vessel may tow signal sources 130 and anothervessel may tow streamers 140, and more or fewer streamers 140 may beused than are shown in FIG. 1, possibly in a variety of different towingconfigurations.

Signal sources 130, in various embodiments, are configured to emit a setof signals (e.g., sound waves) measurable by pressure and accelerationsensor systems 105 located on streamers 140. Sources 130 may include avariety of seismic sources, such as marine vibrators or air guns. Assuch, sources 130 may transmit sound waves into the water, the echoes ofwhich may be detected by systems 105 of the streamers 140. Sources 130may also include a variety of electromagnetic (EM) sources, such asantennas or magnetic coils. Accordingly, sources 130 may transmit EMsignals into the water, which may be detected by the streamers 140.

Pressure and acceleration sensor system 105, in various embodiments, isconfigured to measure the pressure induced by the signals emitted bysources 130 after they have reflected off the geological formationsbelow the seabed. In such an embodiment, system 105 is also configuredto measure the particle acceleration experienced by streamers 140—e.g.,the particle acceleration that is induced by the signals emitted fromsources 130. In some embodiments, system 105 is coupled to electronicequipment (referred to as “geophysical electronic equipment”) aboardvessel 110 that may be used to analyze geophysical data, such as signalsmeasured by system 105 in order to, for example, identify geologicalformations indicative of oil and/or natural gas deposits as part of aseismic survey. Components within system 105 will now be described inmore detail with respect to FIG. 2.

Turning now to FIG. 2, a block diagram of a pressure and accelerationsensor system 105 is shown. In the illustrated embodiment, system 105includes a piezoelectric sensor pair 200, calibration circuits 210, andamplifiers 220. As shown, sensor pair 200 includes two sensors 202A and202B, each having a piezoelectric ceramic 204 and a respective brasshousing 206 attached to that ceramic 204. While ceramics 204 are shownand discussed below, in various embodiments, other forms ofpiezoelectric materials may be used such as piezoelectric crystals(e.g., quartz, Rochelle salt, lead titanate, langasite, etc.),biological matter, etc. Similarly, in some embodiments, a differentmaterial other than brass may be used for housing 206 such as beryllium,copper, bronze, etc. In various embodiments, system 105 is implementeddifferently than shown—e.g., system 105 may include amplifiers 220, butnot calibration circuits 210, etc.

Sensor pair 200, in various embodiments, is configured to producevoltages indicative of the acceleration and/or the pressured experiencedby piezoelectric ceramics 204 (and more generally the streamer 140 thatincludes that pair 200). In various embodiments, ceramics 204 arepiezoelectric materials that produce voltages in response to beingstressed by sound waves causing pressure deviation and particleacceleration or by another applied force, which may come from, forexample, currents moving through the water pushing streamers 140,orientation members (e.g., steerable birds configured to maintain aparticular feather angle) adjusting streamers 140, modules cleaningstreamers 140, etc. Accordingly, ceramics 204 may produce voltages whenbending in response to a change in pressure or an acceleration in adirection having a vector perpendicular to a face of ceramic 204.

The ability of ceramics 204 to produce voltages when stressed may resultfrom their physical piezoelectric properties. Accordingly, ceramics 204may have an asymmetric atomic arrangement that leads to the formation ofelectric dipole moments. When not being stressed, ceramics 204 may beelectrically neutral. But, when a stress is applied to ceramics 204, theatomic arrangement may be deformed causing an electric potentialdifference to exist on the surfaces of ceramics 204. This electricpotential difference may generate a current flowing to circuits 210 vialines 205—the manner of which is discussed below with respect to FIG. 3.

Calibration circuits 210, in various embodiments, are configured tocorrect an imbalance between ceramics 204 in which ceramics 204 ofdifferent sensors 202 produce different voltages when experiencing thesame pressure and/or acceleration. Accordingly, circuits 210 may adjustsensors 202A and/or 202B such that voltages produced by sensor 202A aresimilar to voltages produced by sensor 202B in response to the sameapplied stress. As will be discussed below with FIG. 4, in someembodiments, circuits 210 are voltage dividers coupled to sensors 202.

Amplifiers 220, in various embodiments, are configured to amplifyvoltages received from calibration circuits 210 via circuit lines 215.As will be discussed with FIG. 5, in various embodiments, amplifiers 220may include two differential amplifiers configured to amplifydifferential signals received via lines 215A and 215B and via lines 215Cand 215D. As will be discussed with FIG. 6, in some embodiments, thesedifferential signals may be amplified by amplifiers 220 and combined ina manner that enables separate derivation of pressure and theacceleration experienced by system 105.

Turning now to FIG. 3A, a block diagram of a wiring scheme 300 ofsensors 202 for suppressing noise caused from acceleration is shown. Inthe illustrated embodiment, sensors 202 are connected such that avoltage is generated in response to pressure 310 while a voltage that isgenerated in response to acceleration 320 is attenuated. As shown,pressure exposure 305A depicts the exposure of sensors 202 to a pressure310, and acceleration exposure 305B depicts the exposure of sensors 202to an acceleration 320.

As shown in exposures 305, housings 206A and 206B are coupled to theexternal faces of ceramics 204A and 204B, respectively. Sensor lines205A and 205C are connected to housings 206, respectively, and areconnected together at node 330A. Likewise, sensor lines 205B and 205Dare shown as being connected to the internal faces of ceramics 204 andas being connected together at node 330B. In some embodiments, thewiring of sensors 202 may be implemented differently.

In pressure exposure 305A, sensors 202 are exposed to a pressure 310,which causes ceramics 204 to bend inward in this example. As shown, thismay result in a positive charge appearing on the convex side of the bendand a negative charge appearing on the concave side. These charges, inturn, may result in a voltage difference between nodes 330, the voltagedifference being indicative of an amount of pressure 310 being appliedto sensors 202.

In acceleration exposure 305B, sensors 202 are exposed to anacceleration 320, which causes ceramics 204 to bend toward the left forexample. A shown, this may result in positive charges being present onthe external surface of ceramic 204A and the internal surface of ceramic204B, and may result in negative charges being present on the internalsurface of ceramic 204A and the external surface of ceramic 204B. Due tothe configuration of lines 205, however, these charges cancel out,resulting in little or no voltage difference between nodes 330. Thus,with wiring scheme 300, a voltage difference indicative of acceleration320 is attenuated, while a voltage difference indicative of pressure 310is preserved.

Turning now to FIG. 3B, a block diagram of a wiring scheme 301 ofsensors 202 for suppressing the signal caused from pressure alone isshown. In the illustrated embodiment, sensors 202 are connected suchthat a voltage is generated in response to acceleration 320, while avoltage generated in response to pressure 310 is attenuated.

As shown in exposures 305, brass housings 206 are coupled to ceramics204 in a manner similar to that of FIG. 3A. In the embodiment of FIG.3B, however, line 205A is connected to the internal face of ceramic204A, and line 205B is connected to housing 206A—i.e., lines 205A and205B are swapped relative to the embodiment of FIG. 3A. Lines 205C and205D, in some embodiments, may be changed instead of lines 205A and205B.

As shown, deformations of ceramics 204 responsive to exposures 305 maycreate positive and negative charges in locations to those discussed inrelation to FIG. 3A. Due to the different configuration of lines 205,however, a voltage difference indicative of pressure 310 is attenuatedin wiring scheme 301 instead of a voltage difference indicative ofacceleration 320. In particular, in pressure exposure 305A of wiringscheme 301, the charges cancel out, resulting in little or no voltagedifference between nodes 330, while in acceleration exposure 305B ofwiring scheme 301, these charges may result in a voltage differencebetween nodes 330, the voltage difference being indicative of an amountof acceleration 320 being applied to sensors 202.

Turning now to FIG. 4, a block diagram of a piezoelectric sensor pair200 coupled to calibration circuits 210A and 210B is shown. As mentionedabove, calibration circuits 210 may be configured to calibrate sensors202 in order to correct an imbalance between the voltages produced byceramics 204 in different sensors 202. In the illustrated embodiment,circuit 210A includes a variable resistor 410A and a resistor 415A, andcircuit 210B includes a variable resistor 410B and a resistor 415B. Insome embodiments, pair 200 and circuits 210 are implementeddifferently—e.g., only one calibration circuit (e.g., circuit 210A) iscoupled to sensor pair 200, resistor 415A may be swapped with variableresistor 410A, etc.

Individual ceramics 204 may not always produce the same voltages whenexposed to the same pressure 310 or acceleration 320 due to, forexample, imperfections in the ceramics. In some instances, this mismatchbetween ceramics 204 may result in incorrect readings and may complicatedetermining pressure and acceleration. To correct this issue, in someinstances, mass may be added to (or removed from) ceramics 204. In otherinstances, it may be more beneficial to use calibration circuits 210.

Calibration circuits 210, in various embodiments, adjust the voltagesproduced by sensors 202 to remove imbalances. In the illustratedembodiment, calibration circuits 210 are voltage dividers, which includea variable resistor 410 and a static resistor 415 placed in series. Invarious embodiments, variable resistors 415 may be adjusted in order toadjust the differential voltage appearing on circuit lines 215. As anexample, when ceramic 204A is producing a higher voltage, variableresister 410A may be increased to produce a greater voltage drop acrossresistor 410A and thus lower the differential voltage on circuit lines215A and 215B.

In various instances, calibration circuits 210 may be calibrated priorto being placed in a streamer 140. In some cases, sensors 202 may beexposed to an acceleration 320 using a shaker table. The differentialvoltages measured on lines 215A-B and lines 215C-D may be compared.Based on the degree of mismatch between the differential voltages, insome embodiments, a resistance value is calculated for at least one ofthe variable resistors 410 such that the differential voltages match. Insome instances, sensors 202 may be exposed to a pressure 310 using anacoustic chamber, and a similar process may then be performed tocalibrate circuits 210.

Turning now to FIG. 5, a block diagram of an array of piezoelectricsensor pairs 200 coupled to amplifiers 220 is shown. In the illustratedembodiment, array 510 includes multiple sensor pairs 200, each of whichincludes a ceramic 204A coupled to the inputs of amplifier 220A, and aceramic 204B coupled to the inputs of amplifier 220B. As shown,amplifiers 220 are each coupled to various resistors configured toaffect voltage gains applied to differential voltages produced byceramics 204. While not shown, in various embodiments, each sensor pair200 may be coupled to a calibration circuit 210, which is coupled to theinputs of amplifiers 220A and/or 220B such as discussed below with FIG.6. In various embodiments, amplifiers 220 and array 510 may beimplemented differently—e.g., the connections of a particular ceramic204 to the inputs terminals of a particular amplifier 220 may bereversed.

Array 510, in various embodiments, is a series or group of sensor pairs200 that are configured to measure pressure and acceleration. Eachsensor pair 200 may be calibrated prior to being placed in array 510. Insome embodiments, streamers 140 may have multiple arrays 510 placedalong their length. A given set of two arrays 510 in a streamer 140 maybe oriented with a 90 degree angle (or approximately 90 degree angle)between them such that one array 510 measures acceleration in a firstdirection (e.g., vertical) and the other array 510 measures accelerationin a second direction (e.g., horizontal). Said differently, two arrays510 may be oriented orthogonal to each other so that they measure thevertical and horizontal components of acceleration 320.

Amplifiers 220, in various embodiments, are configured to amplify thedifferences between two input voltages received via lines 215.Accordingly, amplifier line 225A is a voltage representative of thedifference between lines 215A-B, and amplifier line 225B is a voltagerepresentative of the difference between lines 215C-D.

Amplifiers 220 may be calibrated prior to being placed in a streamer140. In various embodiments, an adjustment is made to amplifiers 220such that the voltages resulting from them approximately match. In someembodiments, array 510 and amplifiers 220 may be placed in an acousticchamber and exposed to pressure 310. The voltages measured at lines 225may be compared. Based on the degree of mismatch between the voltages,an adjustment to amplifiers 220 and/or their outputs may be determinedsuch that the voltages match. For example, a software program may applya pre-determined adjustment to the measurements made on lines 225. Insome cases, the adjustment may be made to a balance of amplifiers 220.In other cases, the resistors (shown as R₁, R₂, . . . , R₈) coupled toamplifiers 220 may be selected to cause the voltages on lines 225 tomatch.

Turning now to FIG. 6, a block diagram of a system 105 is shown. In theillustrated embodiment, system 105 includes a sensor pair 200 coupled tocalibration circuits, which themselves are coupled to amplifiers 220.While not shown, system 105 may include multiple sensor pairs 200, eachone being coupled to respective calibration circuits 210, which arecoupled to amplifiers 220. That is, system 105 may include array 510. Insome embodiments, system 105 may be implemented differently thanshown—e.g., circuit line 215A is coupled to the positive terminal ofamplifier 220B and line 215B is coupled to the negative terminal.

System 105, in various embodiments, is configured to produce indications630 that are indicative of pressure 310 and acceleration 320 experiencedby system 105. In some embodiments, the voltage produced by amplifier225A is subtracted from the voltage produced by amplifier 225B (or viceversa) at block 610. The difference between the voltages measured atamplifier lines 225 may provide an indication 630A of acceleration 320.In some embodiments, the voltage produced by amplifier 225A is added tothe voltage produced by amplifier 220B at block 620. The addition of thetwo voltages measured at lines 225 may provide an indication 630B ofpressure 310. In an embodiment in which the wiring to at least one ofthe amplifiers 220 is reversed (e.g., the two inputs voltages areswapped at the terminals), the difference between the voltages measuredat lines 225 is indicative of pressure and the addition of the voltagesis indicative of acceleration. In various embodiments, particle velocityis determined by integrating the measured acceleration.

Turning now to FIG. 7, a block diagram of a method 700 is shown. Method700 is one embodiment of a method performed by a computer system (orother entity) to calibrate sensors (e.g., sensors 202). In someembodiments, method 700 is performed prior to the sensors being placedon a streamer and may be performed using a shaker table.

Method 700 begins in step 710 with the computer system detecting a firstvoltage (e.g., a differential voltage measured between circuit lines215A-B) produced by a first piezoelectric material (e.g., ceramic 204A)in a hydrophone when the hydrophone is exposed to an acceleration (e.g.,an acceleration 320). The hydrophone may be exposed to an accelerationin the absence of pressure (e.g., pressure 310). In step 720, thecomputer system detects a second voltage (e.g., the differential voltagemeasured between circuits lines 215C-D) produced by a secondpiezoelectric material (e.g., ceramic 204B) in the hydrophone when thehydrophone is exposed to the acceleration.

In various embodiments, the computer system detects a third voltage(e.g., the differential voltage measured between amplifier lines 225A-B)produced by a first amplifier (e.g., amplifier 220A) coupled to thehydrophone when the hydrophone is exposed to a pressure (e.g., pressure310). In such embodiments, the computer system detects a fourth voltage(e.g., the differential voltage measured between lines 225C-D) producedby a second amplifier (e.g., amplifier 220B) coupled to the hydrophonewhen the hydrophone is exposed to the pressure. The first and secondamplifiers may be coupled to a plurality of other hydrophones (e.g.,sensor pairs 200 within array 510) that have respective first and secondpiezoelectric materials. In some embodiments, the first amplifier iscoupled to ones of the respective first piezoelectric materials and thesecond amplifier is coupled to ones of the respective secondpiezoelectric materials.

In step 730, the computer system compares the first voltage and thesecond voltage. In some embodiments, step 730 further includes thecomputer system comparing the third voltage and the fourth voltage.

In step 740, the computer system determines, based on the comparing, aresistance for a variable resistor (e.g., resistor 410A or 410B) coupledto one of the first and second piezoelectric materials. In variousembodiments, the resistance is determined such that the variableresistor adjusts a first subsequent voltage that is produced by thefirst piezoelectric material to match (e.g., a match within fivepercent) a second subsequent voltage produced by the secondpiezoelectric material when the hydrophone is exposed to a subsequentacceleration. In various embodiments, the computer system, based on thecomparing of the third and fourth voltages, determines an adjustment forat least one of the first and second amplifiers. The adjustment may bedetermined such that a third subsequent voltage produced by the firstamplifier matches a fourth subsequent voltage produced by the secondamplifier when the hydrophone is exposed to a subsequent pressure (inabsence of acceleration in various cases). In various embodiments, thevariable resistor is included in a voltage divider circuit (e.g.,calibration circuits 210A) coupled to the first piezoelectric material.The voltage divider circuit may be configured to adjust the voltagesproduced by the first piezoelectric material. In various embodiments,method 700 includes additional steps such as performing at least onemeasurement of an acceleration and a pressure being experienced by ahydrophone (e.g., system 105) while a streamer having the hydrophone isbeing towed behind a survey vessel (e.g., vessel 110).

Turning now to FIG. 8, a block diagram of a method 800 is shown. Method800 is one embodiment of a method performed by a computer system (orother entity) to calibrate sensors (e.g., sensors 202) that areconfigured to measure pressure (e.g., pressure 310) and acceleration(e.g., acceleration 320). Method 800, in some embodiments, is performedprior to the sensors being placed on a streamer (e.g., streamer 140) andmay be performed using a shaker table.

Method 800 begins in step 810 with first and second piezoelectricmaterials (e.g., ceramics 204A and 204B) in a hydrophone being subjectedto an acceleration without being subjected to a pressure. The first andsecond piezoelectric materials being subjected to the acceleration maycause them to produce a voltage signal representative of pressure. Thefirst and second piezoelectric materials, in some embodiments, aresubjected to a pressure without being subjected to an acceleration. Thissubjection to a pressure may cause the first and second piezoelectricmaterials to produce a voltage signal representative of acceleration.

In step 820, a sensitivity to acceleration of at least one of the firstand second piezoelectric materials is adjusted such that the voltagerepresentative of pressure is reduced. Adjusting the sensitivity toacceleration may include determining an adjustment to a mass of at leastone piezoelectric material. Adjusting the sensitivity may also includedetermining a resistance for a voltage divider network (e.g.,calibration circuits 210) such that the network matches a sensitivity toaccelerations of the first piezoelectric with a sensitivity toacceleration of the second piezoelectric material. In some embodiments,a balance between first and second amplifiers (e.g., amplifiers 220)coupled to the first and second piezoelectric materials is adjusted suchthat a voltage signal representative of acceleration is reduced. In someembodiments, method 800 includes additional steps such as measuring atleast a pressure and an acceleration experienced by first and secondpiezoelectric sensors (e.g., sensors 202A-B) while the geophysicalequipment (e.g., streamer 140) that includes them is being towed by asurvey vessel (e.g., vessel 110).

Turning now to FIG. 9, a block diagram of a method 900 is shown. Method900 is one embodiment of a method performed by a computer system (orother entity) to calibrate sensors (e.g., sensors 202) that areconfigured to measure pressure (e.g., pressure 310) and acceleration(e.g., acceleration 320).

Method 900 begins in step 910 with first and second voltages beingdetected that result respectively from first and second piezoelectricsensors (e.g., sensors 202) being subjected to an acceleration. In someembodiments, third and fourth voltages are detected that resultrespectively from the first and second piezoelectric materials beingsubjected to a pressure. In step 920, the first voltage and the secondvoltage are compared. The third and fourth voltage may also be compared.

In step 930, based on the comparing in step 920, an adjustment for avoltage divider (e.g., calibration circuits 210) that is coupled to oneof the first and second piezoelectric sensors is determined such thatthe voltage divider causes voltages produced respectively by the firstand second piezoelectric sensors to match when the sensors are subjectedto acceleration. In embodiments in which third and fourth voltages aredetected, based on the comparing in step 920, an adjustment for a firstamplifier (e.g., amplifier 220A) coupled to one of the first and secondpiezoelectric sensors is determined such that the first amplifier causesvoltages produced respectively by the first and second piezoelectricsensors to match when the first and second piezoelectric sensors aresubjected to pressure.

Turning now to FIG. 10, a block diagram of a method 1000 is shown.Method 1000 is one embodiment of a method performed by a system (e.g.,sensor system 105) to measure acceleration (e.g., acceleration 320) andpressure (e.g., pressure 310). In some embodiments, method 1000 isperformed after the system is placed on a streamer.

Method 1000 begins in step 1010 with a first pair of wires (e.g., sensorlines 205A-B) of the system producing a first differential voltage. Insome embodiments, one of the wires (e.g., 205A) in the first pair iscoupled to a first housing structure (e.g., brass housing 206A) includedin a first hydrophone sensor (e.g., sensor 202A) of the system. Theother wire in the first pair (e.g., 205B) may be coupled to a firstpiezoelectric material (e.g., ceramic 204A) that is included in thefirst hydrophone sensor.

In step 1020, a second pair of wires (e.g., sensor lines 205C-D) of thesystem produces a second differential voltage. In some embodiments, oneof the wires (e.g., 205C) in the second pair is coupled to a secondhousing structure (e.g., brass housing 206B) that is included in asecond hydrophone sensor (e.g., sensor 202B) of the system. The otherwire in the second pair (e.g., 205D) may be coupled to a secondpiezoelectric material (e.g., ceramic 204B) included in the secondhydrophone sensor. In some embodiments, a first voltage divider (e.g.,circuit 210A) of the system adjusts the first differential voltageprovided by the first pair of wires. The first voltage divider mayinclude a first variable resistor (e.g., resistor 410A) configured tofacilitate adjustment of the first differential voltage. In someembodiments, a second voltage divider (e.g., circuit 210B) of the systemadjusts the second differential voltage provided by the second pair ofwires. The second voltage divider may include a second variable resistor(e.g., resistor 410B). The first and the second variable resistors maybe used to remove an imbalance between voltages that are generated bythe first and the second piezoelectric materials.

In step 1030, the system determines, based on the first and the seconddifferential voltages, a pressure and an acceleration experienced by thefirst and the second hydrophone sensors. In some embodiments, a firstdifferential amplifier (e.g., amplifier 220A) of the system is coupledto the first voltage divider and is configured to amplify the firstdifferential voltage. In some embodiments, a second differentialamplifier (e.g., amplifier 220B) of the system is coupled to the secondvoltage divider and is configured to amplify the second differentialvoltage. In various embodiments, the system determines the accelerationby determining a difference between the first and the seconddifferential voltages. In various embodiments, the system determines thepressure by adding the first and the second differential voltages.

Turning now to FIG. 11, a block diagram of a method 1100 is shown.Method 1100 is one embodiment of a method performed by a system (e.g.,system 105) to measure both acceleration (e.g., acceleration 320) andpressure (e.g., pressure 310). The system may be configured to measurepressure and acceleration while being towed in a streamer behind aseismic vessel.

Method 1100 begins in step 1110 with first and second piezoelectricmaterials (e.g., ceramics 204A-B) providing voltages in response tobeing stressed. In some embodiments, the first and second piezoelectricmaterials are included in first and second hydrophone sensors (e.g.,sensors 202A-B), respectively.

In step 1120, a calibration circuit (e.g., circuit 210A) adjusts avoltage produced by the first piezoelectric material to match a voltageproduced by the second piezoelectric material when the first and secondpiezoelectric materials are subjected to an acceleration (e.g., 320). Insome embodiments, the calibration circuit is a voltage divider networkthat is coupled to the first piezoelectric material. In someembodiments, an amplifier circuit (e.g., amplifier 220A) adjusts avoltage that results from the calibration circuit to match a voltageassociated with the second piezoelectric material when the first andsecond piezoelectric materials are subjected to a pressure. In variousembodiments, the voltages provided in response to being stressed areindicative of an acceleration and a pressure experienced by the firstand second piezoelectric materials.

Turning now to FIG. 12, a block diagram of a method 1200 is shown.Method 1200 is one embodiment of a method performed by a system (e.g.,sensor system 105) to measure acceleration (e.g., acceleration 320) andpressure (e.g., pressure 310).

Method 1200 begins in step 1210 with a first amplifier (e.g., amplifier220A) coupled to first piezoelectric sensors in a set of hydrophonesensor pairs providing a first voltage (e.g., amplifier output 225A). Insome embodiments, the first amplifier is configured to produce the firstvoltage such that the first voltage is a differential between a voltageproduced by a first piezoelectric material included in a firstpiezoelectric sensor and a voltage produced by a second piezoelectricmaterial included in the first piezoelectric sensor. In step 1220, asecond amplifier (e.g., amplifier 220B) coupled to second piezoelectricsensors in the set of hydrophone sensor pairs provides a second voltage(e.g., amplifier output 225B.) In step 1230, an acceleration and apressure are derived based on the first and second voltages (e.g., viaadd block 620 and subtract block 610).

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of this application (or an application claimingpriority thereto) to any such combination of features. In particular,with reference to the appended claims, features from dependent claimsmay be combined with those of the independent claims and features fromrespective independent claims may be combined in any appropriate mannerand not merely in the specific combinations enumerated in the appendedclaims.

The invention claimed is:
 1. A method, comprising: detecting a firstvoltage produced by a first piezoelectric material in a hydrophone whenthe hydrophone is exposed to an acceleration; detecting a second voltageproduced by a second piezoelectric material in the hydrophone when thehydrophone is exposed to the acceleration; comparing the first voltageand the second voltage; based on the comparing, determining a resistancefor a variable resistor coupled to one of the first and secondpiezoelectric materials, wherein the first piezoelectric material iscoupled across the variable resistor and an in-series resistor includedin a voltage divider circuit, wherein the voltage divider circuit isconfigured to adjust voltages produced by the first piezoelectricmaterial; adjusting the variable resistor to have the determinedresistance such that a first subsequent voltage produced by the firstpiezoelectric material is adjusted to match a second subsequent voltageproduced by the second piezoelectric material when the hydrophone isexposed to a subsequent acceleration; and detecting a third voltageproduced by a first differential amplifier coupled to the variableresistor and the in-series resistor.
 2. The method of claim 1, furthercomprising: detecting the third voltage when the hydrophone is exposedto a pressure; detecting a fourth voltage produced by a seconddifferential amplifier coupled to the hydrophone when the hydrophone isexposed to the pressure; comparing the third voltage and the fourthvoltage; and based on the comparing of the third and fourth voltages,determining an adjustment for at least one of the first and seconddifferential amplifiers.
 3. The method of claim 1, wherein thehydrophone is one of a plurality of hydrophones included in a streamerto be towed behind a survey vessel.
 4. The method of claim 2, whereinthe adjustment is determined such that a third subsequent voltageproduced by the first differential amplifier matches a fourth subsequentvoltage produced by the second differential amplifier when thehydrophone is exposed to a subsequent pressure.
 5. The method of claim2, wherein the first and second differential amplifiers are coupled to aplurality of other hydrophones that have respective first and secondpiezoelectric materials, wherein the first differential amplifier iscoupled to ones of the respective first piezoelectric materials and thesecond differential amplifier is coupled to ones of the respectivesecond piezoelectric materials.
 6. A method, comprising: detecting firstand second voltages resulting respectively from first and secondpiezoelectric sensors being subject to an acceleration; comparing thefirst voltage and the second voltage; based on the comparing,determining an adjustment for a voltage divider coupled to one of thefirst and second piezoelectric sensors such that the voltage dividercauses voltages produced respectively by the first and secondpiezoelectric sensors to match when the first and second piezoelectricsensors are subjected to the acceleration, wherein the adjustment forthe voltage divider modifies a resistance of a variable resistorincluded in the voltage divide, wherein the first piezoelectric sensoris coupled across the variable resistor and an in-series resistorincluded in the voltage divider; and detecting a third voltage producedby a first differential amplifier coupled to the variable resistor andthe in-series resistor.
 7. The method of claim 6, further comprising:detecting third and fourth voltages resulting respectively from thefirst and second piezoelectric sensors being subjected to a pressure;comparing the third voltage and the fourth voltage; and based on thecomparing of the third and fourth voltages, determining an adjustmentfor the first differential amplifier such that the first differentialamplifier causes voltages produced respectively by the first and secondpiezoelectric sensors to match when the first and second piezoelectricsensors are subjected to the pressure.
 8. The method of claim 6, whereinthe first and second piezoelectric sensors are included in geophysicalequipment to be towed by a survey vessel.
 9. The method of claim 6,wherein the voltage divider is coupled to the first piezoelectric sensorvia a first wire coupled to a housing structure associated with thefirst piezoelectric sensor and a second wire coupled to the firstpiezoelectric sensor.
 10. The method of claim 7, wherein a seconddifferential amplifier is coupled to the second piezoelectric sensor,wherein a difference between voltages resulting respectively from thefirst and second differential amplifiers is indicative of a particularacceleration experienced by the first and second piezoelectric sensors.11. The method of claim 10, wherein an addition of the voltagesresulting respectively from the first and second differential amplifiersis indicative of a particular pressure experienced by the first andsecond piezoelectric sensors.
 12. A method, comprising: subjecting firstand second piezoelectric materials in a hydrophone to an accelerationwithout subjecting the first and second piezoelectric materials to apressure, wherein the subjecting to the acceleration causes the firstand second piezoelectric materials to produce a voltage signalrepresentative of pressure; and adjusting a sensitivity to accelerationof at least one of the first and second piezoelectric materials suchthat the voltage signal representative of pressure is reduced, whereinthe adjusting of the sensitivity to acceleration includes determining aresistance for a variable resistor in a voltage divider network suchthat the voltage divider network matches a sensitivity to accelerationof the first piezoelectric material with a sensitivity to accelerationof the second piezoelectric material; wherein the first piezoelectricmaterial is coupled across the variable resistor and an in-seriesresistor included in the voltage divider network; and wherein a firstdifferential amplifier is coupled to outputs of the variable resistorand the in-series resistor.
 13. The method of claim 12, furthercomprising: subjecting the first and second piezoelectric materials to apressure without subjecting the first and second piezoelectric materialsto an acceleration, wherein the subjecting to the pressure causes thefirst and second piezoelectric materials to produce a voltage signalrepresentative of acceleration; and adjusting a balance between thefirst differential amplifier and a second differential amplifier coupledto the first and second piezoelectric materials such that the voltagesignal representative of acceleration is reduced.
 14. The method ofclaim 12, wherein the adjusting of the sensitivity to accelerationincludes determining an adjustment to a mass of the at least onepiezoelectric material.
 15. The method of claim 12, wherein the firstand second piezoelectric materials are subjected to an accelerationusing a shaker table.
 16. The method of claim 12, wherein the hydrophoneis one of a plurality of hydrophones included in a streamer to be towedbehind a survey vessel.